The North Stand at Celtic Park apparently holds over 26,000 seats and therefore is the biggest in Britain. Old Trafford North Stand is the second largest. No idea about the rest of the world, I'm guessing the side stand of the Nou Camp will be the biggest, although whether it's actually a 'stand' or part of the 'bowl' is open to debate.

Waiting for a Bovril in the East Stand upper whilst the 12 year olds struggle with the concept of "service" Lots of standing involved
Trying to get out of the car park before midnight (ok you can sit)
Trying to take back an ORA shirt to the club shop when it says L but in fact it is the L version of a Brazilian Pygmy tribe (Huge amount of standing involved)